The present invention relates to a signal detection method of a phase-change optical disk, and more particularly, to a signal detection method of a phase-change optical disk which can greatly improve a degree of modulation of a reproduction signal and enhance reliability of the reproduction signal.
Generally, a laser beam is irradiated on an optical disk in order to reproduce an information signal recorded on a phase-change optical disk. An angle of reflection with respect to the beam irradiated on the optical disk is varied due to a difference of the complex refractive index of each phase by a phase-change between a crystal state and an amorphous material formed on a recording layer of the optical disk. An optical pickup which is used in conjunction with a phase-change optical disk is designed to convert light which reaches a light receiving element (of the optical pickup) into an electric signal and to reproduce a recorded information signal, using an optical characteristic of a recording layer of the phase-change optical disk.
An operation of optical pickup with respect to a general phase-change optical disk is described with reference to FIG. 1. A semiconductor laser 1 emits a laser beam. A collimation lens 2 collimates the laser beam emitted from the semiconductor laser 1 in the form of a parallel light beam. The laser beam emitted in the form of the parallel light beam by the collimation lens 2 is incident to a quarter-wave plate 4 via a beam splitter 3. The beam incident to the quarter-wave plate 4 is in a plane-polarized state, and is converted into a circularly polarized beam by the quarter-wave plate 4. The circularly polarized beam is incident to an objective lens 5, and the objective lens 5 focuses the incident beam on an information recording surface of an optical disk 7. A beam reflected from the information recording surface of the optical disk 7 is incident to the quarter-wave plate 4 via the objective lens 5. The quarter-wave plate 4 converts the reflected beam into a plane-polarized beam to emit the plane-polarized beam, and a polarized plane of the emitted beam is rotated by 90.degree. with respect to the incident beam. The beam emitted from the quarter-wave plate 4 is reflected from the beam splitter 3 and is incident to a photodetector 6. The photodetector 6 receives the light reflected from the beam splitter 3 and converts the received light into an electric signal.
A construction of the photodetector 6 is shown in detail with reference to FIG. 2. A general photodetector 6 is composed of four-divided light receiving regions 6a, 6b, 6c and 6d. That is, the reflected light from the optical disk is incident to the four light receiving regions 6a, 6b, 6c and 6d of the photodetector 6. When light quantities of the reflected light which reach the respective four light receiving regions 6a, 6b, 6c and 6d are called I1, I2, I3 and I4, respectively, the total of the light quantities of the reflected light which reaches the respective light receiving regions 6a, 6b, 6c and 6d, which is I1+I2+I3+I4, is detected as a reproduction signal.
FIG. 3 is an enlarged view showing an arrangement of a track 8 and marks 9 of a phase-change optical disk, and particularly shows the arrangement of the marks 9 on the track 8 when a signal is recorded on the phase-change optical disk by mark length recording.
However, in the case of detecting information recorded by the mark length recording using the above signal detection method, a difference in amplitude between a signal having a longer mark and a signal having a shorter mark in the reproduced signal becomes large, and thus there is high probability that an error occurs in reproducing a signal. Practically, when detecting a signal by the above method, it has been known that a degree of modulation in the reproduced signal is about 30%. FIG. 4 shows a waveform diagram of a reproduction signal detected by a general method, which shows a low modulation degree.
FIGS. 5A, 5B and 5C are views schematically showing states that laser beams are located at a front portion, an intermediate portion and a rear portion of the mark 9, respectively, and FIGS. 6A, 6B and 6C are views showing distribution of light quantities corresponding to the states when the laser beam is respectively positioned as shown in FIGS. 5A, 5B and 5C. FIGS. 6A and 6C show that the laser beam is out of the center of the mark, and it can be seen that the distribution of light quantities is asymmetrical from the right and left. This phenomenon causes a problem to lessen reliability of a reproduction signal. Particularly, such a problem is remarkably severe when a signal is detected from an optical disk recorded with a high density, i.e., an optical disk having a narrow interval between marks.